Ultrasonic detection systems often include a transducer for generating acoustic signals and a lens which concentrates the signals in a focal region. Such a system is disclosed in Abts U.S. Pat. No. 4,365,515, which is incorporated herein by reference.
When these systems are used to detect very small particulates in fluids passing through the focal region, the noise which reaches the transducer must be kept very low. For example, when a small particulate enters the focal region, it is struck by the ultrasonic signal from the transducer, and the particulate reflects a very small amount of this energy back to the transducer. The transducer detects the small reflection and thus detects the presence of the particulate. Any noise or other interference which reaches the transducer about the same time as a reflection can mask the reflection so that the particulates goes undetected. Damping means for reducing such noise are described in Abts U.S. Pat. No. 4,112,773 and Abts U.S Pat. No. 4,214,484.